Before this commit sunxi_factors_register uses of_iomap(node, 0) to get
the clk registers. The sun6i prcm has factor clocks, for which we want to
use sunxi_factors_register, but of_iomap(node, 0) does not work for the prcm
factor clocks, because the prcm uses the mfd framework, so the registers
are not part of the dt-node, instead they are added to the platform_device,
as platform_device resources.

This commit makes getting the registers the callers duty, so that
sunxi_factors_register can be used with mfd instantiated platform device too.

While at it also add error checking to the of_iomap calls.

This commit also drops the __init function from sunxi_factors_register since
platform driver probe functions are not __init.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>